1. Field of the Invention
This invention relates to a developer carrying member used in a developing apparatus by means of which an electrostatic latent image formed on an electrostatic latent image bearing member such as an electrophotographic photosensitive member or an electrostatic recoding dielectric is developed with a developer to form a toner image in electrophotography, and also relates to a developing apparatus making use of the developer carrying member. This invention still also relates to a developer carrying member whose resin coat layer provided on a substrate of the developer carrying member has been improved, and further relates to a developing apparatus making use of such a developer carrying member.
2. Related Background Art
Conventionally, in electrophotography, copies or prints are obtained by forming an electrostatic latent image on an electrostatic latent image bearing member (photosensitive drum) by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a developer having a toner, to form a toner image, transferring the toner image to a transfer medium such as paper as occasion calls, and then fixing the toner image to the transfer medium by the action of heat, pressure or heat-and-pressure. Developing systems in electrophotography are grouped into a one-component developing system, which requires no carrier, and a two-component developing system, which makes use of a carrier.
The one-component developing system includes a powder cloud method, in which the toner is used in an atomized state; a contact developing method, in which a toner held on a developer carrying member having a flexibility or elasticity is directly brought into contact with the surface of an electrostatic latent image bearing member to perform development; and a jumping developing method, in which the toner is not brought into direct contact but the toner is caused to fly toward the surface of an electrostatic latent image bearing member by the action of an electric field formed between the electrostatic latent image bearing member and the developer carrying member. A contact one-component developing method or a one-component jumping developing method is commonly used.
Developing apparatus employing the one-component developing system have advantages that they require no carrier and require no mechanism for controlling the concentration of toners and carriers and hence the developing assemblies themselves can be made compact and light-weight.
As toners used in such a developing system, toners with small particle diameter are recently used so that electrophotographic apparatus can be made digital and can be made much higher in image quality. For example, in order to improve resolution and character sharpness to reproduce electrostatic latent images faithfully, toners having a weight-average particle diameter of about 4 to 10 μm are used. It is demanded, for the purpose of more reducing power consumption of apparatus from the viewpoint of ecology, to lower fixing temperature of toners in order to improve fixing performance of the toners, or, for the purpose of making electrophotographic apparatus more compact and light-weight, to improve transfer efficiency of toners in order to reduce waste toner. In order to improve fixing performance of toners, glass transition temperature (Tg) of binder resins used in the toners are made lower, or low-molecular weight components are made larger in proportion in molecular weight distribution of binder resins. Also, in order to improve anti-offset properties of toners, a method is known in which a wax capable of improving plasticity of binder resins is added to toner particles. Still also, in order to improve transfer efficiency of toners, a method is known in which a transfer efficiency improver having an average particle diameter of 0.1 to 3 μm and a hydrophobic silica fine powder having a BET specific surface area of 50 to 300 m2/g are added to toner particles, or in which toner particles are spherical-treated by mechanical impact force.
As a first method for controlling charge quantity of toners, it is prevalent to add a charge control agent to toner particles. However, dyes or pigments used as charge control agents have a tendency of adhering to various members when added to toner particles in a large quantity.
As a second method for controlling charge quantity of toners, a method is proposed in which a suitable material is used in triboelectric charge-providing members so as to make toners have proper charge quantity.
In the developing apparatus employing the one-component developing system, the toner comes into contact with a developer carrying member and a developer layer thickness control member when it is passed through the part between the developer carrying member and the developer layer thickness control member so as to be made into a thin layer, and hence these members have a great influence on making the toner have proper charge quantity. In particular, in the case of a developing apparatus employing a magnetic one-component developing system, which makes use of a magnetic toner, the magnetic toner moves on the developer carrying member by the action of a magnetic force of a magnet built in the developer carrying member, and hence the magnetic toner is frequently rubbed against the developer carrying member. Accordingly, the selection of materials for the developer carrying member has a great influence on the charging performance of the magnetic toner.
As developer carrying members used in the one-component developing system, commonly used are, in the contact developing method, one in which an elastic member of urethane rubber, EPDM rubber, silicone rubber or the like is molded on a shaft made of a metal such as stainless steel, and one in which a layer of an elastomer is formed on the surface of a cylindrical member of aluminum or stainless steel. In this case, the elastic member is incorporated therein with components such as a plasticizer, a vulcanizing agent, a release agent and a low-molecular weight component. It is proposed to provide a barrier layer or a protective layer on the layer surface of the elastic member so that these components can be prevented from bleeding out of the elastic member to affect members adversely. It is further proposed to form at the outermost surface a surface layer using a resin using a material having good release properties or using a resin having good charge-providing properties to toners.
As disclosed in Japanese Patent Applications Laid-Open No. H02-105181 and No. H03-036570, proposed is, as a developer carrying member (developing sleeve) used in a non-contact one-component developing method, a developing sleeve comprising a developing-sleeve substrate on the surface of which a resin coat layer is formed in which a conductive material such as carbon black or graphite and a solid lubricant stand dispersed in a binder resin having good charge-providing properties. However, the surface profile of the developing sleeve has a great influence. Hence, if the surface profile of the developing sleeve has changed as a result of repeated use, the coat level of the toner can not easily be made stable, and the developing performance tends to become unstable. A sufficient performance may be achievable in low-volume process cartridges, which are not required to have durability (running performance) so much. However, in the case of high-volume process cartridges, which are required to have a high durability, the surface profile of the developing sleeve may greatly change because of scrape of the resin coat layer to tend to result in a great change in toner's coat level as well. Such a change in coat level of the toner has an influence also on the chargeability of the toner because the frequency of rubbing between the toner and the developing sleeve changes.
As disclosed in Japanese Patent Application Laid-Open No. H03-200986, a developing sleeve is proposed to the surface of which spherical fine particles have been added to form unevenness on the developing-sleeve surface. Such a method in which spherical particles are added is a good means in order to form a surface profile with uniform unevenness and make stable the coat level of the toner. However, when the developing sleeve is repeatedly used over a long period of time, or in a developing method in which a strong stress is applied to the surface of the developing sleeve, the use of spherical resin particles as the spherical fine particles may cause scrape during repeated use over a long period of time to make the resin coat layer of the developing sleeve have a low surface roughness, so that the coat level of the toner may decrease and also the melt adhesion of toner tends to occur.
As disclosed in Japanese Patent Application Laid-Open No. H08-240981, a developing sleeve is proposed in which conductive spherical particles having a true density of 3 g/cm3 or less have been added to a resin layer of the developing sleeve to form unevenness on the surface of the developing sleeve. Such a developing sleeve makes stable the coat level of the toner and also the conductive spherical particles themselves haves a good wear resistance. Hence, the stress applied to the toner between the developing sleeve and the developer layer thickness control member is relaxed to bring an improvement in durability of the resin coat layer itself. However, at resin portions present between conductive spherical particles, the scrape may selectively progress because of the repeated use over a long period of time and the rubbing with the toner, so that the resin coat layer may change in surface roughness to therefore tend to cause a change in coat level of the toner.
In the developing apparatus employing the one-component developing system, it is long awaited to provide a developing sleeve whose resin coat layer which forms the surface layer of the developing sleeve has been more improved.